The synthesis of both anti- and syn-benzo(a)pyrene diolepoxides (BPDE) has been achieved in our laboratory. The stability of the N2-and N7-guanine products of reaction of anti-BPDE with DNA has been investigated. Consideration of the relationship between carcinogenicity and the theoretical reactivity indices of polycyclic aromatic hydrocarbons has led to a publication in "Cancer Research". Metabolism of the l2 mono-methylbenz(a)anthracenes by a liver microsomal fraction has been studied using HPLC to separate the metabolites, which were identified by their fluorescence spectrum. Mutation studies in V79 cells with a series of labelling methylating carcinogens has related quantitatively the yield of mutants with the extent of reaction of the carcinogen with the 0 to the 6th power-position of guanine. Toxicity however was related to the overall extent of DNA reaction. Transformation of hamster embryo cells in culture has been achieved with benzo(a)pyrene. The transformed cells have yielded tumors in animals. Growth of these tumors in vitro is being studied and their properties compared with in vitro transformants, and with cells derived from tumors induced in hamsters by a single injection of benzo(a)pyrene. It has been established that the promoting agent, TPA, is an effective inhibitor of metabolic co-operation between mutant and non-mutant cells in dense culture.